In order to maximize the commercial application of coal combustion fly ash as a component in concrete manufacture, many methods have been developed to remove carbon particles from the fly ash, thereby reducing the drawbacks related to the presence of undesirable amounts of carbon.
Such methods to remove carbon particles typically use means based on froth flotation (see U.S. Pat. No. 6,068,131), particle size distribution U.S. Pat. No. 5,996,808 or electrostatic techniques (see U.S. Pat. No. 5,938,041, JP2004243154A, JP2005279489A). However, although these methods may be appropriate to reduce the carbon content of the beneficiated fly ash in order to meet the specifications for application in cement or concrete, they do not allow reducing the carbon content to such a low value (e.g. less than 2%) to ensure that the concrete systems will not be negatively impacted by the remaining carbon particles. Furthermore, these techniques are quite expensive in term of investment and yield elevated operational costs (energy, pre and post processing).
Various combustion methods to remove carbon from fly ash have been disclosed (U.S. Pat. No. 5,390,611, US 2004231566, US 2006180060). One of the most widely used combustion method is the carbon burn out using fluidised beds (see U.S. Pat. No. 5,399,194, U.S. Pat. No. 5,160,539, US 2008075647, US 2008173217 or WO 2007097745). However, this method presents the following drawbacks. Fluidized beds cannot maintain fluidized conditions without the fine particles being transported. Therefore, such methods require an additional system to capture the small particles on top of the normal collection equipment and there are restrictions regarding the size distribution of the fly ash particle to be beneficiated by such methods so that the number of entrained small particles is limited.
Furthermore, when using a bubbling bed, temperature control of the bubbling bath has the consequence to partially melt the fly ash particles when temperatures are increased too much. A very high residential time at elevated temperature may damage the glassy fly ash particles, resulting in partial sintering or agglomeration, therefore reducing the quality of the final beneficiated product or requiring post processes like further grinding or milling that increase the complexity and the costs of the process. Reducing the bed temperature may circumvent the problem of overheating but limits the burning out of the carbon particles and results in fly ash having still a high LOI. Sometimes, the bed temperatures are controlled using a recirculation loop of beneficiated fly ash particles to avoid that the temperatures increases too much. However, such a recirculation loop increases the technical complexity of the installation. Furthermore, fly ash particles having low heat value are almost impossible to treat with those techniques.
Most of these problems are related to the use of fluidized beds since sintering of the particles due to important and local overheating may occur even at lower overall bed temperature when auto-combustion occurs.
Moreover, GB 1 577 234 discloses a method and an installation for treating fly ash for the production of bricks. Preheated fly ash particles are injected in a combustion chamber wherein an air stream carries them upwardly. The mean residence time of the fly ash particles is approximately 5 seconds. The combustion of the fly ash particles is therefore a rapid combustion. Carbon dust is injected in the combustion chamber as soon as the carbon content of the fly ash particles drops below 3%.
However, the method does not allow assuring the spontaneous combustion of all the carbon present in the fly ash particles. Moreover, the combustor temperature control strategy does not allow avoiding completely overheating of the fly ash and the fly ash particles thus obtained have modified properties and reduced quality. Indeed, the temperature in the combustor is not controlled accurately. As an example, the fly ash feed rate and/or the carbon feed rate are not controlled or used to control the combustor temperature. Furthermore, the energy balance of the system is controlled measuring the temperature outside the combustor, at the discharge duct and it does not assure that some parts of the reactor have a lower temperature causing a carbon content increase in the beneficiated fly ash particles or a higher temperature causing fly ash overheating. Moreover, coarse particles of carbon are settled at the conical bottom of the combustor wherein they start burning causing the problem of increasing the temperature and forming melted particles of fly ash.
Furthermore, the amount of air injected to the combustor is the stoichiometric amount. Therefore, any variation in the carbon content can produce an incomplete combustion, which may generate carbon monoxide. Afterburning equipment is located at the exit of the combustion tube to complete the carbon oxidation to carbon dioxide. However, this afterburning step is dangerous due to the explosive condition of producing carbon monoxide.